Due to the spread of small electronic equipment many attempts have been made in recent years to achieve high-density packaging of electric circuits. Thus, due to the increase in the exothermic density of electronic equipment, axial or mixed-flow fans are used to cool the equipment.
In a conventional fan assembly, an axial fan 1 is placed in such a manner as to provide an appropriate space between blade tips of the fan and the inner circumferential surface of an annular wall 2, as shown in FIG. 20, so that in a blowing state in which a motor section 3 is powered on, the axial fan 1 rotates around a shaft 4 to cause an air flow 5 from a suction side to a discharge side. In this blowing state, however, the speed of the air flow increases on the suction side of the tips of fan blades 8, and the energy of the air flow is converted into a pressure energy. Consequently, inter-blade secondary flows occur at the trailing edges of the blades to create low-energy areas at these edges. In this part of the fan assembly, a large loss is likely to occur to release the flow, and in such a case, the air flow leaves a plate surface and vortexes occur in this area. As a result, turbulence noise may increase to degrade the noise level and the static pressure-air quantity characteristic (hereinafter referred to as the "P-Q characteristic"). This phenomenon is frequently observed particularly if the discharge side is subjected to flow resistance (system impedance) to cause more leaking vortexes at the blade tips, thereby stalling the axial fan.
In order to improve the characteristics of such an axial fan, the shape of the annular wall provided at the outer circumference of the axial fan has been improved in the fan assemblies described in Japanese Patent Application No. 8-174042, Japanese Patent Application No. 9-151450 and Japanese Patent Application No. 9-260738 all assigned to the applicant. These fan assemblies are shown in FIGS. 21 to 23 wherein annular plates 7a to 7e are provided in a casing body 9 as the annular wall 2 encompassing the axial fan 1. The annular plates 7a to 7e are laminated via spacers 13, and a slit 6 is formed between each pair of adjacent annular plates 7a to 7e. In a blowing state, this configuration allows air to be sucked into the annular wall 2 through the slits 6 provided between the annular plates 7a to 7e, in order to restrain occurrence of leaking vortexes at the blade tips as well as rotating stall, thereby improving the P-Q characteristic and reducing noise. In addition, National Publication of International Patent Application No. 6-508319 and U.S. Pat. No. 5,292,088 describe such fan assemblies that comprise a plurality of ring bodies arranged at the outer circumference of the axial fan at intervals so that air vortexes flowing through the gaps between the ring bodies increase the flow rate of the fluid. Alternatively, U.S. Pat. No. 5,407,324 describes a fan assembly wherein the inner circumferential portions of annular plates encompassing the outer circumference of the axial fan are inclined along the direction of the wind and wherein these annular plates are accumulated so as to form a plurality of stages in order to enable air to flow between the inner and outer circumferences of the annular wall.
Although these inventions all improve the characteristics of the axial fan by sucking air from the outer circumference of the axial fan, they describe only the configuration of the ring bodies (annular plates) provided at the outer circumferential portion of the axial fan but do not particularly describe the shape of the axial fan. Thus, to make most of the characteristics of the axial fan, the shape of the fan must be adjusted to the annular wall. The shape of the axial fan has been generally improved by cutting the blades of the axial fan in their cylindrical surfaces concentric with a rotating shaft of the axial fan, developing the cylindrical surfaces to be replaced by a planar infinite linear blade series, applying to this blade series the linear blade profile series theory established for airplanes and the like in order to predict performance or to determine a three-dimensional shape suitable for operating conditions.
FIGS. 24 to 29 show the shapes of conventional axial fans by way of examples. As shown in FIGS. 26 and 27, a cross section of a conventional axial fan 1 obtained by cutting it in a way of forming a cylinder concentric with the rotating shaft is in such a form that wing-shaped blades 8 are joined together in the radial direction. This is because the air flows in the radial direction of the axial fan 1 are ignored in designing the conventional axial fan. According to this design, calculated and actual values have not significantly deviated from each other if the axial fan has an annular wall that prevents air from flowing in from the outer circumference and if it is operated with a relatively low air flow resistance. In addition, in order to improve the characteristics of the axial fan when the air flow resistance is slightly high, an advancing blade is used in which the chord center line of the blade is inclined at a specified angle in the rotating direction, as shown in FIGS. 28 and 29. In FIG. 24, a thin line h is an iso-thickness line denoting the thickness of the blade, an alternate long and short dash line i is a chord center line obtained if the blade is cut in a concentric cylindrical surface, and a broken line k denotes the position of the maximum thickness obtained if the blade is cut in a concentric cylindrical surface. When this conventional axial fan is used in combination with the casing 9 with the slits provided in the annular wall therein, the air flows on the blades of the axial fan flow in the directions shown by the arrows in FIG. 24. FIG. 25 shows the blade, which has been cut in the cross section shown by alternate long and two short line a--a' that extends along this air flow. In FIG. 25, the neighborhood of the blade tip s is formed to be thicker to some degree, so air flows flowing onto this part collide against the surface of the blade tip and the air layer is released near both edges t1 of the tip. In addition, the distribution of the blade thickness, on which the blade performance significantly depends, substantially deviates from an ideal blade shape arrangement, so the blade shape cannot be expected to contribute to effecting a lift. The air layer is likely to be released at the trailing edge t2, thereby degrading the characteristics of the axial fan.
An invention that does not suck air from the outer circumference of the annular wall but that attempts to improve the characteristics of the axial fan by improving the shape of the blade tip is the impeller described in Japanese Patent Laid-Open No. 6-307396 wherein the aerodynamic force is improved while noise is reduced by configuring the cross section of the outer circumferential blade tip so as to include a single-side curved shape located at the leading edge and having projecting curves only on the pressure surface side; and a circular shape portion contiguous to the single-side curved shape. In addition, Japanese Patent Laid-Open No. 8-121391 describes an electric fan that reduces aerodynamic noise by folding the outer circumference of the blade into a curve. Alternatively, Japanese Patent Application Laid-Open No. 8-284884 describes a fluid machine wherein the outside of the tip of a moving blade is removed over a specified height from its tip end to form a thinner portion of a specified thickness at the inside of the tip in order to reduce the leakage of a fluid through the tip clearance, thereby improving the efficiency of an axial fan. It is premised that these conventional techniques for the shape of the axial fan, however, require to provide an annular wall preventing air from flowing in from its outer circumference, so sufficient characteristics cannot be obtained by applying such blade shapes to a configuration for sucking air from the outer circumference of the annular wall, as described above.
An invention that requires air inflow through slits provided in the outer circumference of the axial fan in order to optimize the shape of the axial fan is the fan assembly in Japanese Patent Application No. 9-260738 assigned to the applicant and shown in FIGS. 29 to 33. In FIG. 30, a thin line h is an iso-thickness line denoting the thickness of a blade, an alternate long and short dash line i is a chord center line obtained if the blade is cut in a concentric cylindrical surface, and a broken line k denotes the position of the maximum thickness in a cross section obtained by cutting the blade at a concentric cylindrical surface. FIG. 31 shows the blade, which has been cut in the cross section shown by an alternate long and two short line a-a' that extends along the air flow. As shown in FIG. 29, among sweepforward angles .theta.1 to .theta.3, the sweepforward angle .theta.3 at the blade tip is formed to be larger than the two others. In other words, the blade is formed by folding the blade tips in the rotating direction. This configuration enables air flows flowing in through the slits to be smoothly taken in to improve the P-Q characteristic of the fan assembly. Furthermore, the blade is shaped in such a way that as the blade tip approaches, the position of the maximum thickness in a cross section obtained by cutting the blade in a concentric cylindrical surface gradually moves backward toward the trailing edge of the blade. Specifically, the cross sections of the blade along lines 1.sub.1 -1.sub.1 ', 1.sub.2 -1.sub.2 ', 1.sub.3 -1.sub.3 ', m-m', and n-n' shown in FIG. 32 are shaped as shown in FIGS. 33(a) to (e), respectively. Reference numeral F denotes the position of the maximum thickness. As shown in FIG. 31, this shape maximizes the blade shape effect even on air flows flowing in from the outer circumference of the annular wall and allows air flowing in through the slits to flow smoothly at the blade tip. Furthermore, according to this shape, the blade shape effect also serves to cause a lift acting on air flows flowing in from the blade tip or the air layer is restrained from being released at the trailing edge to enable the air flows flowing in through the slits to be effectively converted into an air capacity, thereby further improving the P-Q characteristic of the fan assembly.
An object of the present invention is to further improve the blade shape of the fan assembly that sucks air into the annular wall through the slits provided in these walls as in Japanese Patent Application No. 9-260738, thereby improving the aerodynamic performance or energy efficiency.